Television receivers are known which feature simultaneous display of pictures, e.g., picture-in-picture (PIP) or picture outside picture (POP) formats. In the picture in picture receivers an inset or "small" picture to be displayed within an area of a "main" or "large" picture is subjected to vertical and horizontal compression by means of selective storage and retrieval from a memory and the compressed picture video signal is inserted within an area of the main picture video signal by means of a multiplex switched that is controlled by timing signals provided by the picture in picture compression processor.
Examples of picture in picture compression processors are described, for example, by D. L. McNeely and R. T. Fling in U.S. Pat. No. 4,890,162 which issued Dec. 26, 1989 and by E. D. Romesburg in U.S. Pat. No. 4,768,083 which issued Aug. 30, 1988. Picture outside picture (POP) processing is similar except that the auxiliary video signal is combined with the main video signal to produce side by side images when displayed.
Non-linear video signal processors are known wherein a video signal is subjected to non-linear amplification within selected portions of the video signal range for providing improved detail in displayed images. O. H. Shade, for example, describes a non-linear video processor featuring both so-called "black stretch" and "white stretch" processing for improving detail in the darker and lighter areas of displayed images in U.S. Pat. No. 2,760,008 which issued Aug. 21, 1956.
It has been recognized as being desirable to provide both picture in picture processing and non-linear video signal processing (particularly "black stretch" processing) in a television receiver. Such a receiver is described by Lineberry in U.S. Pat. No. 5,202,765 entitled TELEVISION RECEIVER WITH PICTURE IN PICTURE AND NON-LINEAR PROCESSING which issued Apr. 13, 1993. In this receiver a form of non-linear (e.g., black stretch) processing is applied to a main video signal after the insertion of the compressed auxiliary video signal by the multiplex switch that combines the main and compressed video signals for display. Such a configuration provides a number of advantages over systems employing non-linear processing before picture-in-picture insertion for reasons discussed in detail by Lineberry in the patent.
The Lineberry system (100) is illustrated in block diagram form in FIG. 1 herein and comprises a picture in picture ("PIP" hereafter) processor 106 which receives main (S1) and inset (S2) picture video signals from respective sources (102 and 104). Processor 106 compresses the inset picture video signal S2 vertically and horizontally and includes a multiplex switch (not shown) that inserts the compressed inset picture video signal within the main picture video signal (S1) by means of time division multiplexing to provide luminance S3 and chrominance S4 output signals of PIP format. The luminance component S3 is applied to a non-linear processor 108 (e.g., a black stretch, white stretch or similar processor) to provide a non-linearly processed luminance PIP signal S5. The chrominance signal S4 and the non-linearly processed luminance signal S5 are then applied to a video display processor 110. This processor provides additional processing functions such as hue and saturation control, brightness and contrast control, RGB matrixing, etc. to the PIP video signal and the resultant processed video signal S6 is applied (e.g., in RGB component form) to a kinescope or other suitable display device 112. A PIP timing signal S7 is applied to an inhibit input of the non-linear processor 108 to enable the processor when the main picture signal S1 is being displayed and to disable the processor when the inset picture signal S2 is being displayed. As a result, the non-linear processing is applied to displayed images only in the main picture area and not in the inset picture area of the displayed images.
Advantageously, as explained by Lineberry, restricting non-linear processing to the main picture area is of particular significance in certain signal conditions in systems where nonlinear processing (e.g., black stretch) is applied subsequent to picture in picture insertion. Specifically, in such cases the insert picture black level may be modulated as the black stretch circuit dynamically adjusts for changes in the main picture scene content. In instances where the insert picture consists of relatively low IRE signal and the main picture consists of relatively high IRE signals, the insert picture can be "stretched" so far towards black level that its blacks are clipped and much of the detail lost. To a lesser extent, the black level of the main picture can be modulated as the black stretch circuits adjust for changing scene content in the insert picture. Because the insert picture is small in comparison to the main picture, this effect is not as great as the one noted above where the main picture is the predominant controlling factor. Accordingly, by disabling the non-linear processing when the inset picture is displayed, the luminance levels of the inset picture are not disturbed by the level of the main picture signal.